1. Field of the Invention
This invention relates to a device for reading an image (an image reading device), and more particularly to a device for reading an image (an image reading device) having photoelectric conversion devices arranged in a transverse direction to an original, which is usable for a facsimile machine, an image reader, a digital copying machine and the like.
2. Description of the Prior Art
As this type of image reading apparatus as described above have been conventionally known three types of image reading devices such as a non-contact type, a close contact type and an entirely-close contact type. Recently, in association with requirement for a compact and lightweight type of facsimile machine, image reader, digital copying machine or the like, the image reading apparatus itself is required to be compact and lightweight. This requirement causes the close contact and entirely-close contact types to be mainly propagated in the market. However, a more compact and lightweight of image reading device has been further required for these types.
In addition, an image which is read out by the image reading device is required to be outputted with image quality of high precision and high gradation at high speed, and thus an image reading device having high photosensitivity and photoresponsivity has been also required.
Further, a photoconductor or photodiode type has been generally known as a photoelectric conversion device which is used in the image reading device. The photoconductor type allows a large amount of current to flow therethrough, but has low photoresponsivity. Inversely, the diode type permits only a small amount of current to flow therethrough, but has very excellent photoresponsivity. Recently, the photodiode type has been mainly utilized to satisfy the requirement for obtaining high gradation.
As an example of the diode type serving as the photoelectric conversion device has been well know a semiconductor structure in which PN or PIN junction is formed on a silicon substrate or an insulating substrate with monocrystal silicon semiconductor, polycrystal silicon semiconductor or amorphous silicon semiconductor. A conventional photoelectric conversion device is designed as a best device so that semiconductor layers having a PN or PIN junction are laminated on a monocrystal silicon substrate or an insulating substrate and positive and negative electrodes are formed on the top and bottom surfaces of the semiconductor layers in a laminating direction of the semiconductor layers. Further, the junction surface of the PN or PIN junction has been conventionally designed substantially in parallel with the principal plane of the semiconductor layers or the substrate to irradiate a large amount of light onto the junction surface.
As described above, since the conventional photoelectric conversion device has the junction surface formed in parallel with the principal plane of the semiconductor layers or the substrate, an electric field which is generated inside of the semiconductor layers through the PN or PIN junction is directed perpendicularly to a light-irradiating surface (plane) and thus the intensity of the irradiated light is inhomogeneous in the direction of the electric field. As a result, it is difficult to effectively generate electrons/holes in the conventional photoelectric conversion device and output them to the outside.
Further, the photoelectric conversion device using amorphous semiconductor has a disadvantage that it is difficult to effectively take out carriers generated in the semiconductor by light irradiation to the outside because the semiconductor material has a short diffusion length.
As described above, miniaturization, high photoresponsibility and effectivity of photosensitivity have been insufficient to the conventional image reading device, and thus a further development has been required for the conventional image reading device. In addition, a lower cost has been also required for this type of image reading device.